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C-Peptide

Test Overview

A C-peptide test measures the level of this
peptide in the blood. It is generally found in amounts equal to
insulin because insulin and C-peptide are linked when first made by the
pancreas. Insulin helps the body use and control the amount of sugar (glucose)
in the blood. Insulin allows glucose to enter body cells where it is used for
energy. The level of C-peptide in the blood can show how much insulin is being
made by the
pancreas. C-peptide does not affect the blood sugar
level in the body.

A C-peptide test can be done
when
diabetes has just been found and it is not clear
whether
type 1 diabetes or
type 2 diabetes is present. A person whose pancreas
does not make any insulin (type 1 diabetes) has a low level of insulin and
C-peptide. A person with type 2 diabetes can have a normal or high level of
C-peptide.

A C-peptide test can also help find the cause of low
blood sugar (hypoglycemia), such as excessive use of medicine to
treat diabetes or a noncancerous growth (tumor) in the pancreas (insulinoma).
Because man-made (synthetic) insulin does not have C-peptide, a person with a
low blood sugar level from taking too much insulin will have a low C-peptide
level but a high level of insulin. An insulinoma causes the pancreas to release too much insulin, which
causes blood sugar levels to drop (hypoglycemia). A person with an insulinoma
will have a high level of C-peptide in the blood when they have a high level of insulin.

Why It Is Done

A C-peptide test is done to:

Help tell the difference between type 1
diabetes and type 2 diabetes.

Find the cause of low blood sugar
(hypoglycemia).

Check to see whether a tumor of the pancreas
(insulinoma) was completely removed.

How To Prepare

Your doctor will give you instructions about eating and drinking before this test.

Insulin and
some oral medicines used to treat type 2 diabetes can change the test results.
Your doctor may ask you to stop these medicines before your blood test.

Talk to your doctor about any concerns you have regarding the need for
the test, its risks, how it will be done, or what the results will mean. To
help you understand the importance of this test, fill out the
medical test information form(What is a PDF document?).

How It Is Done

The health professional drawing your
blood will:

Wrap an elastic band around your upper arm to
stop the flow of blood. This makes the veins below the band larger so it is
easier to put a needle into the vein.

Clean the needle site with
alcohol.

Put the needle into the vein. More than one needle stick
may be needed.

Attach a tube to the needle to fill it with
blood.

Remove the band from your arm when enough blood is
collected.

Put a gauze pad or cotton ball over the needle site as
the needle is removed.

Put pressure on the site and then put on a
bandage.

How It Feels

The blood sample is taken from a vein in
your arm. An elastic band is wrapped around your upper arm. It may feel tight.
You may feel nothing at all from the needle, or you may feel a quick sting or
pinch.

Risks

There is very little chance of a problem from
having a blood sample taken from a vein.

You may get a small bruise at the site. You can
lower the chance of bruising by keeping pressure on the site for several
minutes.

In rare cases, the vein may become swollen after the blood
sample is taken. This problem is called phlebitis. A warm compress can be used
several times a day to treat this.

Results

A C-peptide test measures the level of
this peptide in the body.

Normal

The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

The
level of C-peptide in the blood must be read with the results of a blood
glucose test. Both these tests will be done at the same time. A test to measure insulin level also may be done.

High values

A high level of
C-peptide with a low blood glucose level may mean that an insulin-producing tumor of
the pancreas (insulinoma) is present or that the use of certain medicines such
as sulfonylureas (for example, glyburide) is causing the high level.

If C-peptide levels are high after an insulinoma is taken out,
it may mean that the tumor has returned or that the tumor has spread to other
parts of the body (metastasized).

Low values

Low levels of both C-peptide and blood
glucose are found in liver disease, a severe infection,
Addison's disease, or insulin therapy.

A
low level of C-peptide with a high blood glucose level is found in people with
type 1 diabetes.

Complete removal of the
pancreas (pancreatectomy) causes a C-peptide level so low it can't be
measured. The blood glucose level will be high, and insulin will be needed in
order for the person to survive.

What Affects the Test

Reasons you may not be able to
have the test or why the results may not be helpful include:

Taking medicines, such as insulin, or sulfonylurea
medicines for type 2 diabetes.

Having kidney failure. Both insulin and C-peptide are
removed from the body by the kidneys. C-peptide levels may be high in a person
with kidney failure.

Being
obese. More insulin is made in obese people and can
cause high levels of C-peptide.

What To Think About

A C-peptide test must be done at the same time
as a blood glucose test. To learn more, see the topic
Blood Glucose.

A person with new type 2
diabetes often has a normal or high level of C-peptide in the blood. Over time,
a person with type 2 diabetes may develop a low level of C-peptide.

To help tell the difference between type 1 and type 2 diabetes and
to help guide treatment, most doctors look at a person's age, weight, and how
long symptoms have been present. In rare cases, a C-peptide stimulation test
may be done to help tell the difference between the two types of diabetes.
During a C-peptide stimulation test, a blood sample is taken to measure
C-peptide. Then a shot of a hormone to increase blood sugar (glucagon) is given
into a vein in the arm. Another blood sample is taken. In people with type 1
diabetes, C-peptide levels will be low because the pancreas cannot make any
insulin in response to the glucagon. In people with type 2 diabetes, C-peptide
levels will be higher than the first blood test because the pancreas is making
more insulin in response to the glucagon.

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